As the recent electronic devices, such as portable telephone and PDA (personal digital assistant), use more and more electronic components, the power sources for these devices need to cope with a broad range of output voltage and load current. As an example of series regulator power-supply circuit that forms such a power source, an LDO (low dropout) voltage regulator (linear regulator) employing a MOSFET (hereinafter referred to as MOS transistor) for an output control transistor (path transistor) is used to realize low dropout, as disclosed in, for example, JP-A-2002-258954.
FIG. 8 is a circuit diagram showing a conventional series regulator 90. The numerals placed near each transistor in FIG. 8 represent the number of units and the size of the transistor (m (units)×W (gate width) [μm]/L (gate length)[μm]). These numerals will be described in detail later. The series regulator 90 has an output control transistor M91 that receives a power supply voltage VIN90 input and outputs a stable output voltage VOUT90. It also has voltage resistors R91 and R92. It also includes a differential input stage including, NMOS transistors M92, M93, and M95, and PMOS transistors M96 and M97. It also has an NMOS transistor M94. The PMOS transistors M96 and M97 form a current mirror circuit and the NMOS transistors M94 and M95 form a current mirror circuit. The current mirror circuit formed by the NMOS transistors M94 and M95 is a constant-current circuit that supplies a current proportional to a current I90 to the differential input stage. The NMOS transistors M92 and M93, which form the differential pair of the differential input stage, also can be referred to as source-coupled pair because the sources of the two transistors are connected to each other.
A load resistance RL90 and a capacitance CL90 are connected to an output terminal out90 of the series regulator 90. In this circuit, negative feedback is applied so that an output voltage VOUT90 and a reference voltage VREF90 has the relation expressed by the following equation (1):VOUT90=(R91+R92)VREF90/R92  (1).
The characteristics of the feedback loop of this series regulator will now be described. Main poles in the feedback loop normally exist at a node X90 and a node Y90. The pole of the node X90 is substantially decided by the load resistance RL90 and the capacitance CL90, and shifts to the high-frequency side as the load resistance RL90 decreases. If the frequency at the pole of the node Y90 is not sufficiently higher than the frequency at the pole of the node X90, for example, if the frequency at the pole of the node Y90 is not higher than the frequency at the pole of the node X90 by two digits (=40 dB/(20 dB/decade)) or more in the case where the DC gain is 40 dB, a phase lag by the node Y90 is superposed on a phase lag by the node X90 at a frequency lower than the UG frequency (frequency at which the open loop gain of the feedback loop is 1 (0 dB)) at the node X90, and the phase margin is extremely reduced, thus lowering stability. Therefore, the frequency must be set at the pole of the node Y90 in accordance with the required output current and DC gain.
The pole of the node Y90 is decided by the product of an output resistance Ry90 of the node Y90 and a capacitance Cy90 substantially equal to the gate capacitance Cgs90 of the output control transistor M91 (total capacitance component connected to the node Y90). Since the size of the device of the output control transistor M91 is decided by the current supply capability of the series regulator, it is difficult to change the size. Therefore, the output resistance Ry90 must be reduced to provide a high frequency at the pole of the node Y90.
FIG. 9 is a circuit diagram showing another conventional series regulator 90a. The same parts as in FIG. 8 are denoted by the same numerals. Here, the series regulator 90a has resistors R96 and R97. Even when it is difficult to adjust the output resistance Ry90 by using a bias current of the output control transistor M91 and the channel length L of the device, Ry90 can be adjusted to be approximately equal to R97 by providing the resistors R96 and R97 having equal resistance values. However, if the output resistance Ry90 is reduced, the DC gain of the differential amplification stage decided by the product gm90·Ry90 of the output resistance Ry90 and the transconductance gm90 of the differential pair formed by the transistors M92 and M93 is reduced. For example, if the output resistance Ry90 is reduced by one digit, the transconductance gm90 of the differential pair must be increased by one digit to maintain the DC gain of the differential amplification stage. To increase the transconductance gm90 by one digit, when the volt-ampere characteristic of the transistors M92 and M93 follows the square law and the device size is not to be changed, the bias current must be increased by two digits. Even when the channel width of the transistors M92 and M93 is increased by one digit, the bias current must be increased by one digit.
Specifically, the bias current I90 of each of the transistors M92 and M93 can be expressed by the following equation (2):I90=K(VGS−Vth)2  (2),where K represents the transconductance parameter (constant), VGS represents the gate-source voltage, and Vth represents the threshold voltage of the transistors M92 and M93.
The transconductance gm90 can be expressed by the following equation (3):gm90=dI/dV=2K(VGS−Vth)  (3).Thus, to increase the transconductance gm90 by one digit, (VGS−Vth) must be increased by one digit. According to the equation (2), the bias current I90 is increased by two digits. Therefore, it is difficult to realize lower power consumption.
Although JP-A-2002-258954 discloses realizing lower power consumption, it does not disclose any amplifier circuit that achieves both low output resistance and sufficient DC gain. In terms of this, JP-A-2002-258954 is the opposite because it uses many current mirror circuits.
Accordingly, there still remains a need to improve both low output resistance and sufficient DC gain without increasing the bias current in a series regulator. The present invention addresses this need.